1. Field of the Invention
The invention relates to an automatic transmission that is mounted on a vehicle.
2. Description of Related Art
An invention regarding an automatic transmission is described in US 2012/0283064 A. The automatic transmission described in US 2012/0283064 A includes four planetary gear sets and six engagement mechanisms. The automatic transmission is able to selectively set one of nine forward speed stages and one reverse speed stage. Four of the six engagement mechanisms are friction clutches. The remaining two engagement mechanisms are dog clutches. Each speed stage is set by operating three of those six engagement mechanisms so that the three engagement mechanisms are in an engaged state at the same time. When a shift that accompanies an operation to engage a released one of the dog clutches or an operation to release an engaged one of the dog clutches is carried out, a torque capacity of at least one friction clutch not associated with the shift is increased. Thus, the intended dog clutch is operated in a state where rotation speeds of rotating members that are engaged by the dog clutch are synchronized with each other or in a state where substantially no load is applied between the rotation members.
In the automatic transmission described in US 2012/0283064 A, fixed elements or a coupling relationship among rotating elements of the four planetary gear sets are changed by operating the four friction clutches and the two dog clutches. The nine forward speed stages are allowed to be set by such a change. Because the number of speed stages is large, it is possible to expand the range of gear ratios that are allowed to be set by the transmission. Alternatively, it is possible to reduce a step in gear ratio between the adjacent speed stages.